1. Field of the Invention
The present invention relates to an automatic gain controller (AGC), preferably used for radio communication receivers, and more particularly to an automatic gain controller (AGC) which is capable of performing an adequate gain control on an input signal whose amplitude is steeply changed.
2. Prior Art
The land mobile communication, such as portable telephones and automobile telephones, is generally subjected to severe electric field environment. To assure stable communication under such severe conditions, digital modulation systems and time division multiplexing (TDM) systems have been recently adopted. Furthermore, employment of transmitter antenna hopping and frequency hopping is now examined.
The transmitter antenna hopping is characterized by antenna extraction among a plurality of antennas provided on a transmitter by which an optimum resultant signal is always obtained even if communication conditions are changed or worsened. Similarly, the frequency hopping is characterized by frequency extraction among a plurality of carrier frequencies for optimization of signal receiving condition.
Meanwhile, radio communication receivers are equipped with automatic gain controllers to adequately respond to the change of input signal derived from variations of electric field environment etc.
A typical conventional radio communication receiver, to perform automatic gain control, comprises, as shown in FIG. 3, a receiver antenna 1 detecting a radio frequency band signal, a receiver filter 2 extracting a desired frequency band signal from the detected radio frequency band signal, a first-stage amplifier 3 amplifying the extracted signal, a frequency converter 4 converting an input signal from radio frequency band to intermediate frequency band, a variable gain amplifier 5 amplifying an intermediate frequency band signal, and a gain controller 6 controlling the variable gain amplifier 5.
According to above-described conventional receiver, the antenna 1 receives the radio frequency band signal A sent from a transmitter (not shown), and outputs this radio frequency band signal A to the receiver filter 2. The receiver filter 2 extracts a desired radio frequency band signal B from the input radio frequency band signal A, and outputs this desired radio frequency band signal B to the first-stage amplifier 3. The first-stage amplifier 3 amplifies the extracted radio frequency band signal B, and outputs an amplified signal C to the frequency converter 4. The frequency converter 4 converts the radio frequency band signal C to an intermediate frequency band signal D, and outputs the intermediate frequency band signal D to the variable gain amplifier 5.
The variable gain amplifier 5 amplifies the intermediate frequency band signal D by varying its amplification factor according to a gain control signal F supplied from the gain controller 6, and generates an IF (intermediate frequency) signal E which is delivered to a next-stage circuit (not shown) and the gain controller 6. The gain controller 6 outputs the gain control signal F to the variable gain amplifier 5 to control the amplification factor of the variable gain amplifier 5 in such a manner that the IF signal E is maintained at constant as time averaged value.
Such a conventional automatic gain controller, however, has a problem that, when the input signal level varies steeply, its automatic control function cannot follow this signal change. More specifically, as shown in FIG. 4, when a radio frequency band signal is transmitted from a transmitter employing antenna hopping system or frequency hopping system, the level of an input signal received by a receiver may vary abruptly in response to switching of transmitter antenna or carrier frequency. The above-described conventional automatic gain control apparatus cannot follow such a sudden change of input signal level.
This is because the gain control characteristic itself includes a time constant. Therefore, when the amplitude of desired radio frequency band signal B suddenly decreases at the time the transmitter's antenna is switched from an antenna #1 to an antenna #2, it requires a significant time until the amplitude of the gain control signal F is decreased correspondingly. The amplification factor of the variable gain control amplifier 5 is increased with reducing amplitude of the gain control signal F. As a result, at the moment immediately after the transmitter's antenna is switched to the antenna #2, the variable gain controller 5 fails to amplify the radio frequency band signal B whose amplitude just became small, rather acting to unexpectedly reduce the amplitude of the signal B.
Similarly, when the amplitude of desired radio frequency band signal B suddenly increases at the time the transmitter's antenna is switched from the antenna #2 to the antenna #1, it requires a significant time until the amplitude of the gain control signal F is increased correspondingly. As a result, at the moment immediately after the transmitter's antenna is switched to the antenna #1, the variable gain controller 5 fails to attenuate the radio frequency band signal B whose amplitude just became large, rather acting to unexpectedly increasing the amplitude of the signal B.
Consequently the amplitude range of IF signal generaged from the variable gain amplifier 5 becomes wide, which requires a circuit following the automatic gain controller to have a very large dynamic range. A signal, therefore, will cause a distortion unless a sufficiently large dynamic range is provided in the circuit following the automatic gain controller.
Eliminating this disadvantage by reducing the time constant is not preferable since the automatic gain control function itself is no longer available.